299_2.java

public String getHint(String secret, String guess) {
    //table records the appearance of a digit
    //digit from secret will increase the counter
    //digit from guess will decrease the counter
    int[] count = new int[10];
    
    int counterA = 0, counterB = 0;
    
    for(int i = 0; i < secret.length(); i++){
        int a = secret.charAt(i) - '0', b = guess.charAt(i) - '0';
        if( a == b){
            //accurate match (same digit with same position)
            counterA ++;
        }else{
            
            //if prev part of guess has curr digit in secret
            //then we found a pair that has same digit with different position
            if(count[a] < 0) counterB ++;
            
            //if prev part of secret has curr digit in guess
            //then we found a pair that has same digit with different position                
            if(count[b] > 0) counterB ++;
            
            count[a] ++;
            count[b] --;
        }
    }
    
    return counterA + "A" + counterB + "B";
}